This invention relates to an improved paving material and a method for using the material.
Paving of roadways, sidewalks, pathways and the like is commonly accomplished with asphaltic materials. For example, building of an automobile roadway typically calls for the preparation of an appropriate road bed of fill, crushed stone or the like. Layers or courses of asphalt material are then placed on the road bed. As each layer is placed, it is appropriately leveled and compacted.
Typically the asphalt material is a mixture of bitumen and stone which are heated and mixed together at a site mixing plant. The hot mixture is then transported to the roadway for application by special machinery. The composition of each asphaltic layer as well as the thickness of each layer is dependent upon the projected amount of use and type of use of the roadway. An automobile roadway sublayer may typically have a thickness of seven to twelve inches of course asphalt and a final or top layer on the order of two to three inches in thickness of fine asphalt. On occasion, in order to improve skid resistance, sand or stone chips are provided on or within the surface of the top layer. The entire mass, of course, is leveled and then rolled by road rollers to provide a smooth, firm, level surface.
In addition to the use of asphaltic materials for new roadway construction such materials are useful for roadway repair. Repair o; asphalt roadways is often performed by scraping away an entire layer of the roadway and replacing the removed layer with a new layer of asphaltic material. The new layer may be reconstituted asphalt made in part from previously removed asphalt material. Alternatively, newly prepared asphalt can be used. The patching of asphalt roadways as well as patching of concrete highways may also be effected by placement of a hot asphalt patch in the area which requires repair after that area has been appropriately cleaned.
As an alternative to the use of hot mixtures of asphaltic materials, there are many cold mixes of asphaltic materials used for patching purposes. Thus, the term "asphalt" as used in this application is to be used in its broadest sense and includes any of those paving materials either in cold form or hot form such as bitumen, petrochemical mixtures, polymeric mixtures and other mixtures to pave and repair roadways.
In the repair of roadways or other asphaltic surfaces, it has been found beneficial to apply heat to the area being repaired during or after the repaving or repair operation. For example, a gas flame heater or a radiant heater may be used to heat the asphaltic material and cause that material to soften or to be cured. The heating of the asphaltic material enhances its workability. If the asphalt is to be removed or scarified, heating of the asphalt material will enhance the scarification operation. Moreover, the reheated material may be reconstituted for replacement and reuse on the roadway.
Among the methods which have been proposed for heating of asphalt material is the use of microwave energy. Jeppson in U.S. Pat. No. 4,594,022 as well as in U.S. Pat. Nos. 4,319,856, 4,175,885, 4,252,459 and 4,252,487 describes various methods and apparatus for heating pavements with microwave energy. In U.S. Pat. No. 4,594,022, Jeppson further describes the use of a sheet or layer of microwave reflective material such as a metal foil below a top layer of asphaltic material. The embedded sheet of metallic material acts as a microwave reflector to enhance the heating of the top layer of asphaltic material. This is accomplished by reflection of the microwave energy from the metal foil layer. As a further refinement of the method and construction described by Jeppson, openings may be defined in the sheet for control of passage of microwave energy further into the asphalt.
Jeppson teaches that microwave energy will typically penetrate into a asphalt material approximately seven to eight inches. This is based upon the use of microwave equipment positioned at the surface of a layer which generates microwaves having frequencies of 915 megahertz (MHz) or 2450 MHz. Such microwaves have wave lengths of 33 centimeters and 12 centimeters respectively. As noted by Jeppson, however, the full microwave spectrum includes frequencies from about 400 MHz to about 300,000 MHz corresponding to wave lengths from about 75 centimeters to about 0.1 centimeters.
In sum, in the Jeppson patents there is a disclosure of the concept of using microwave energy to heat various asphaltic materials including hot asphaltic materials as well as cold mixes for the purpose of facilitating paving, repaving, scarifying, patching and other roadway paving and repair operations. There has remained, however, a need for enhancement of the use of microwave energy which is imparted to an asphalt mix or layer of paving. Further, there has remained a need to develop ways to eliminate direct or radiant heating of asphalt materials during repair, etc. Jeppson notes, for example, that the microwave energy source which he teaches is often useful in combination with alternative heat sources. Elimination or diminishment of the need for such alternative heat sources would be beneficial in terms of energy efficiency. Thus, there has remained a need for improved and additional methods for controlling the dispersal and utilization of microwave energy in the repair of asphaltic roadways.